Minimizing grazing incidence reflections for reliable euv power measurements

ABSTRACT

A method includes generating light in a light generating chamber, causing a portion of the generated light to pass through a tube having a roughened inner surface, and detecting the portion of the generated light that has passed through the tube using a photodetector. The roughened inner surface of the tube has a surface roughness sufficient to cause grazing incidences of light to be eliminated rather than to be reflected off the roughened inner surface. In one example, the method includes outputting a signal from the photodetector to a controller, with the signal corresponding to the detected portion of the generated light. The light generated in the light generating chamber can be extreme ultraviolet (EUV) light. In tests using roughened and non-roughened protection tubes, the roughened tube was found to minimize or essentially eliminate the contribution to EUV energy from grazing incidence reflections off the inner surface of the tube.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No.14/463,605, filed Aug. 19, 2014. The disclosure of this priorapplication from which priority is claimed is incorporated herein byreference.

BACKGROUND

Extreme ultraviolet (EUV) light is used in applications such as extremeultraviolet lithography (EUVL). The extreme ultraviolet (EUV) light maybe generated using an EUV source in which a target material isirradiated by a laser source. The irradiation of the target material bya high power laser source leads to the generation of plasma which emitsEUV light. A collector situated in a vessel focuses the photons of theplasma so that the photons are directed out of the vessel and into anEUV consuming system such as an extreme ultraviolet lithography system(EUVL).

To monitor the generation process, EUV sensors can be used to measurethe energy of the EUV light. In vessels provided with multiple EUVsensors, significant EUV energy discrepancies have been observed betweenthe multiple EUV sensors.

It is in this context that embodiments arise.

SUMMARY

In an example embodiment, a light source is provided. The light sourceincludes a light generating chamber in which a collector having areflective surface is disposed. A target material generator configuredto propel a quantity of target material toward an irradiation region isdisposed in front of the reflective surface of the collector. Aplurality of photodetector modules is disposed external to the lightgenerating chamber, with each of the plurality of photodetector modulesbeing directed toward the irradiation region. A plurality of tubes isdisposed between a corresponding photodetector module and theirradiation region. Each of the tubes has a centerline directed towardthe irradiation region, and each of the tubes has a roughened innersurface.

In one example embodiment, the light generating chamber is configured togenerate extreme ultraviolet (EUV) light. In one example embodiment, theroughened inner surface of each of the plurality of tubes has a surfaceroughness sufficient to cause grazing incidences of light to beeliminated rather than to be reflected off the roughened inner surface.

In one example embodiment, the inner surface of each of the plurality oftubes is roughened by bead blasting the inner surface of each of theplurality of tubes, and threading the bead-blasted inner surface of eachof the plurality of tubes.

In one example embodiment, the roughened inner surface of each of theplurality of tubes has a surface roughness in a range from about 20microns to about 1 millimeter. In another example embodiment, theroughened inner surface of each of the plurality of tubes has a surfaceroughness in a range from about 100 microns to about 0.5 millimeter. Inyet another example embodiment, the roughened inner surface of each ofthe plurality of tubes has a surface roughness in a range from about 200microns to about 0.1 millimeter.

In another example embodiment, a method is provided. The method includesgenerating light in a light generating chamber, causing a portion of thegenerated light to pass through a tube having a roughened inner surface,detecting the portion of the generated light that has passed through thetube using a photodetector, and outputting a signal from thephotodetector to a controller, with the signal corresponding to thedetected portion of the generated light.

In one example embodiment, the generating of light in the lightgenerating chamber generates extreme ultraviolet (EUV) light. In oneexample embodiment, the roughened inner surface of the tube has asurface roughness sufficient to cause grazing incidences of light to beeliminated rather than to be reflected off the roughened inner surface.In one example embodiment, the inner surface of the tube is roughened bybead blasting the inner surface of the tube, and threading thebead-blasted inner surface of the tube.

In one example embodiment, the roughened inner surface of the tube has asurface roughness in a range from about 20 microns to about 1millimeter. In another example embodiment, the roughened inner surfaceof the tube has a surface roughness in a range from about 100 microns toabout 0.5 millimeter. In yet another example embodiment, the roughenedinner surface of the tube has a surface roughness in a range from about200 microns to about 0.1 millimeter.

In yet another example embodiment, another method is provided. Thismethod includes generating light in a light generating chamber, causinga portion of the generated light to pass through a tube having aroughened inner surface, and detecting the portion of the generatedlight that has passed through the tube using a photodetector.

In one example embodiment, the generating of light in the lightgenerating chamber generates extreme ultraviolet (EUV) light. In oneexample embodiment, the roughened inner surface of the tube has asurface roughness sufficient to cause grazing incidences of light to beeliminated rather than to be reflected off the roughened inner surface.In one example embodiment, the inner surface of the tube is roughened bybead blasting the inner surface of the tube, and threading thebead-blasted inner surface of the tube.

In one example embodiment, the roughened inner surface of the tube has asurface roughness in a range from about 20 microns to about 1millimeter. In another example embodiment, the roughened inner surfaceof the tube has a surface roughness in a range from about 100 microns toabout 0.5 millimeter. In yet another example embodiment, the roughenedinner surface of the tube has a surface roughness in a range from about200 microns to about 0.1 millimeter.

Other aspects and advantages of the inventions will become apparent fromthe following detailed description, taken in conjunction with theaccompanying drawings, which illustrate by way of example the principlesof the inventions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified schematic view of an extreme ultraviolet (EUV)source.

FIG. 2 shows a cross-sectional view of an EUV photodetector (PD) mountedon the vessel of an EUV source.

FIG. 3 is a schematic diagram that illustrates the operation of an EUVPD module.

FIG. 4A is an isometric view of a protection tube that shows additionaldetails of the protection tube, in accordance with one embodiment of theinvention.

FIG. 4B is a cross-sectional view of a protection tube that showsadditional details of the protection tube, in accordance with oneembodiment of the invention.

FIG. 5 illustrates an ideal model for surface roughness.

FIG. 6A is a photograph of a fluorescent screen when the beam of EUVphotons was passed through the original (non-roughened) protection tubebefore reaching the screen.

FIG. 6B is a photograph of the fluorescent screen when the beam of EUVphotons was passed through the roughened protection tube before reachingthe screen.

FIG. 7 is a flowchart diagram illustrating the method operations thatcan be used to roughen the inner surface of a protection tube, inaccordance with one embodiment of the invention.

FIG. 8 is a flowchart diagram illustrating the method operations used togenerate light, in accordance with one embodiment of the invention.

FIG. 9 is a flowchart diagram illustrating the method operations used tomeasure light energy, in accordance with one embodiment of theinvention.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the exemplary embodiments.However, it will be apparent to one skilled in the art that the exampleembodiments may be practiced without some of these specific details. Inother instances, process operations and implementation details have notbeen described in detail, if already well known.

FIG. 1 shows a simplified schematic view of an extreme ultraviolet (EUV)source. As shown in FIG. 1, EUV source 100 includes vessel 102, whichdefines a light generating chamber. Source laser 104 and optics 106 aresituated outside of vessel 102. Source laser can be any suitable laser,e.g., an excimer laser, a deep ultraviolet (DUV) laser, etc. Optics 106,which includes steering and focusing optics, provides a controlledpathway from source laser 104 to vessel 102. Target material generator(TG) 108 generates target material for irradiation by the source laser104, as will be described in more detail below. As shown in FIG. 1,target material generator 108 generates droplets 114 of target material,e.g., liquid tin. It will be appreciated by those skilled in the artthat target material generator 108 also can generate the target materialin forms other than droplets. For example, the target material can begenerated in the form of a wire, a tape, or a stream. Further, inaddition to tin, other suitable target materials, e.g., xenon andlithium, also can be used. Target material catcher (TC) 110 catches thetarget material that is not irradiated by the source laser. To enablethis functionality, the target material catcher 110 is situated onvessel 102 in an opposing relationship relative to target materialgenerator 108.

In operation, target material from target material generator 108 ispropelled from the target material generator through irradiation region112. When the target material is irradiated by the source laser 104 inirradiation region 112, a plasma is generated. Collector 120 disposedwithin vessel 102 has a reflective surface that collects EUV photons ofthe plasma and focuses the collected EUV photons out of the vessel tofocal point 130, which is the focal point of the collector. The focalpoint 130 coincides with EUV consuming system 132, which uses the EUVlight to perform a function. In one example, UEV consuming system is anEUV lithography system.

To measure the EUV power of the EUV light generated in vessel 102, anumber of EUV photodetector (PD) modules are installed on the outside ofthe vessel. As shown in FIG. 1, six EUV photodetector (PD) modules aredisposed around the outside of vessel 102, and these EUV PD modules arelabeled as PD-1, PD-2, PD-3, PD-4, PD-5, and PD-6. It should beunderstood the use of six EUV PD modules is shown by way of example, andthat number of EUV PD modules can be varied to be either fewer or morethan six to meet the needs of different applications. In one example,two of the EUV PD modules are installed near the top and the bottom ofvessel 102, respectively, and the other four EUV PD modules areinstalled around the side of the vessel. In one example, the top andbottom EUV PD modules are situated on vessel 102 so that they areseparated from one another by 180 degrees. In one example, the four sideEUV PD modules are placed as symmetric pairs that are separated by 180degrees. In other words, the side EUV PD modules forming each symmetricpair are located in a diametrically opposing relationship relative toone another. Further, those skilled in the art will appreciate that FIG.1 shows a projection of a three-dimensional system in two dimensions.Thus, it should be understood that the EUV light path is perpendicularto the plane in which the six EUV PD modules are placed.

Additional details regarding the structure and operation of the EUV PDmodules are described below with reference to FIGS. 2 and 3. Thoseskilled in the art will appreciate that vessel 102 may includeadditional vessel metrology ports and modules that have been omittedfrom the simplified schematic view shown in FIG. 1. For example, vessel102 can be provided with purging sources, vacuum sources,instrumentation for monitoring temperature, a droplet detection module,a droplet illumination module, a nozzle viewing camera, a targetmaterial catcher viewing camera, a light emitting diode, etc.

FIG. 2 shows a cross-sectional view of an EUV PD mounted on the vesselof an EUV source. As shown in FIG. 2, port 140 provides a passagewayfrom the interior of vessel 102 to the exterior of the vessel.Protection tube 142, which is disposed within the passageway defined byport 140, is mounted directly on the port on the vacuum interface. Inone example, protection tube 142 includes a flange section 142 a thatenables the protection tube to be secured in place. The flange section142 a is provided with a number of holes that allow bolts or othersuitable fasteners to be used to secure the protection tube 142 inplace. Additional details regarding the protection tube 142 and theflange section 142 a are described below with reference to FIGS. 4A and4B.

With continuing reference to FIG. 2, gate valve 144 is mounted on theflange section 142 a of protection tube 142. Gate valve 144 is coupledin flow communication with vacuum pump port 145 and gas inlet 147. EUVPD module PD-X is mounted on gate valve 144. EUV PD module PD-X includesa mirror 150 and a sensor 152, the operation of which will be describedbelow with reference to FIG. 3. In one example, sensor 152 is aphotodetector. As shown in FIG. 2, the EUV PD module PD-X includes afirst part in which mirror 150 and sensor 152 are disposed and a secondpart which extends from a side of the first part. A connector plate 154is provided at the end of the second part of EUV PD module PD-X so theEUV PD module can be connected to another structure, e.g.,instrumentation, controls, gas purge, vacuum, etc. The EUV PD modulePD-X shown in FIG. 2 can be used as the EUV PD modules labeled PD-1,PD-2, PD-3, PD-4, PD-5, and PD-6 in FIG. 1.

FIG. 3 is a schematic diagram that illustrates the operation of an EUVPD module. As described above, EUV photons are generated in irradiationregion 112 when the target material flashes to plasma. These EUV photonsdisperse in all directions from irradiation region 112. The total amountof the generated EUV photons includes three portions. A first portion ofthe total amount of the generated EUV photons is collected by thecollector (see reference number 120 in FIG. 1) and directed toward thefocal point of the collector (see reference number 130 in FIG. 1). Asecond portion of the total amount of the generated EUV photons iscollected by the EUV PD modules disposed around the exterior of thevessel, with the second portion being proportional to the total amountof the generated EUV photons. The third portion of the total amount ofthe generated EUV photons is dispersed throughout the light generatingchamber. The dotted line shown in FIG. 3 indicates the path followed bythe EUV photons into EUV PD module PD-X. As shown in FIG. 3, the EUVphotons exit vessel 102 via protection tube 142, which is disposedwithin the wall defining port 140. The EUV photons propagate in theprotection tube 142 and fill the space within the protection tube. TheEUV photons that do not propagate directly through protection tube 142reflect off the inner wall 144 x of the protection tube, as will bediscussed in more detail below.

From the protection tube 142, the EUV photons propagate to the mirror150 in EUV PD module PD-X. The mirror 150, which, as shown in FIG. 3, islocated in the upper left corner of EUV PD module PD-X, reflects the EUVphotons toward the lower right corner of the EUV PD module in whichsensor 152 is disposed. The EUV light energy detected by sensor 152 istransmitted from the sensor by wire 156, which extends from the sensorto the end of EUV PD module PD-X at which connector plate 154 isdisposed.

FIG. 4A is an isometric view of a protection tube that shows additionaldetails of the protection tube. As shown in FIG. 4A, protection tube 142has a cylindrical configuration and includes a flange section 142 a atone end thereof. The flange section 142 a includes six (6) holes 142 hthat enable protection tube 142 to be secured in place with suitablefasteners, e.g., bolts, screws, etc. It will be appreciated by thoseskilled in the art that the number of holes in the flange section can bevaried to suit the needs of particular situations. For example, flangesection 142 a can be provided with three (3) holes, four (4) holes, five(5) holes, etc. Inner wall 142 x defines the hollow space withinprotection tube 142. Protection tube 142 can be made of any suitablemetallic material. In one example, protection tube 142 is made ofstainless steel, e.g. Type 304, Type 304L, Type TP316L stainless steel,etc. The protection tube 142 can be either a seamless cylindrical tubeformed by, e.g., cold drawing, or a cylindrical tube formed by welding.

FIG. 4B is a cross-sectional view of a protection tube that showsadditional details of the protection tube. As shown in FIG. 4B, thecylindrical tube section of protection tube 142 has a length L and adiameter D. It will be appreciated by those skilled in the art that thelength L and diameter D of the cylindrical tube section of protectiontube 142 can be varied to suit the needs of particular situations. Inone particular example, the length L is about 273 mm and the diameter Dis about 15 mm. As used herein, the term “about” means that thespecified parameter can be varied within a tolerance range of ±25%.Theflange section 142 a is joined to the cylindrical tube section ofprotection tube 142 so that a portion of the cylindrical tube section isembedded in the flange section. In one example, about 5 mm to about 10mm of the cylindrical tube section is embedded in the cylindrical tubesection. The flange section 142 a can be joined to the cylindrical tubesection using any suitable joining technique, e.g., welding, etc.

During operation of an EUV source having six EUV PD modules as shown inthe example of FIG. 1, significant EUV energy discrepancy was observedbetween the sensors (photodetectors) of these EUV PD modules. It wasinitially believed that reflections off the inner wall of the protectiontube for EUV photons at grazing incidence to the inner wall of the tube(a small angle relative to the inner wall surface) would not be asignificant factor in the observed energy discrepancy, as the roughnessof the inner wall surface of the protection tubes being used (about 200nm) was approximately 15 times greater than the wavelength of the EUVlight (13.5 nm). Thus, it was initially believed that the EUV lightcontacting the inner wall surface of the protection tube at grazingincidence would be eliminated rather than reflected and therefore wouldnot end up reaching the sensor of an EUV PD module. However, as will beexplained in more detail below, it was unexpectedly determined in asubsequent investigation that grazing incidence reflections were indeeda major contributor to EUV energy discrepancy observed between thesensors of the EUV PD modules.

The complex refractive index of materials for EUV wavelengths leads tototal external reflection at the vacuum-material interface. This impliesthat as the angle of incidence approaches the critical angle ofincidence (the angle of total external reflection), the reflectivity ofthe material surface increases sharply. For small angles of incidencerelative to the inner wall of the stainless steel protection tube, aconsiderable amount of the EUV light will be reflected. In other words,the inner wall surface of the protection tube may work as a channelingguide for EUV light. The amount of EUV light channeled toward the sensorof an EUV PD module will depend on the geometry of the setup, including,by way of example, the diameter and length of the protection tube, theEUV collection solid angle, and the distance from the protection tube tothe sensor (photodetector) of the EUV PD module. The reflectivity of thesurface also depends on the surface flatness for a given wavelength ofincident light and increases for decreasing surface roughness. In thecase of EUV light, a surface roughness on the order of the wavelength,13.5 nm, would be expected for efficient reflection. However, as notedabove, discrepancy between EUV PD modules was found when protectiontubes having a surface roughness on the order of 200 nm, which is about15× larger than the EUV wavelength, were used.

Grazing incidence reflections off the inner wall surface of theprotection tube along the length of the cylindrical wall result inhigher than normal EUV energy measurements because such reflectionseffectively channel EUV light toward the sensor of an EUV PD module. Thepresence of grazing incidence reflections can be verified by observingthe “hot spots” created on the detection surface. In an effort to solvethe problem of grazing incidence reflections off the inner wall surfaceof a protection tube having a surface roughness of about 200 nm, EUVpower measurements were taken during operation of an EUV source havingprotection tubes with a roughened inner wall surface. In thisinvestigation, the inner wall surface of the protection tubes wasroughened using two different roughening techniques. In a firstoperation, the inner wall surface of the protection tube was beadblasted. In a second operation, the bead-blasted inner wall surface wasthreaded. The resulting roughened inner wall surface of the protectiontube was too coarse for measuring the surface figure. As such, it wasnecessary to estimate the surface figure, as explained in more detailbelow.

Roughness produced by machining can be characterized as a combination oftwo independent quantities: (i) ideal roughness (ISR); and (ii) naturalroughness. Ideal roughness (ISR) is a function of feed and geometry. ISRis the best possible finish obtainable for a given tool shape and feed.When calculating ISR, all inaccuracies of the machine tool areneglected. For a sharp tool without a nose radius, the maximum height ofunevenness is given as: Rmax=f/(cot φ+cot β), where, as shown in FIG. 5,“f” is the feed, “φ” is the major cutting edge angle, and “β” is theworking minor cutting edge angle. The surface roughness value is Rmax/4.In a real situation, Ra=0.0321(f²/r), where “r” is the corner radius.

In the case of the roughened protection tubes used in the investigation,an 11/16 fine thread was used to as part of the surface rougheningprocess. The 11/16 fine thread has a feed of 0.0625 inch. According tothe tool supplier, the corner radius (“r”) is 0.01 inch, so the surfaceroughness formula Ra=0.0321(f²/r) yields a value of 318 μm (microns). Asdiscussed above, the surface roughness of the inner wall surface of theoriginal (non-roughened) protection tubes used when the EUV powerdiscrepancy was observed was about 200 nm (nanometers). Thus, the innerwall surface that was roughened by bead blasting and threading has asurface roughness that is more than 1,000 times larger than the surfaceroughness of the original protection tubes.

In tests conducted using the roughened protection tube and the original(non-roughened) protection tube, the roughened protection tube was foundto minimize or essentially eliminate the contribution to EUV energy fromgrazing incidence reflections off the inner surface of the protectiontube. In these tests a special EUV PD module was substituted for one ofthe other EUV PD modules. This EUV PD module included a Ce:YAGfluorescent screen on the port of the EUV PD module. The fluorescentscreen has a Zr outer layer to filter out-of-band emission from theplasma. Thus, in the tests, the beam of EUV photons generated in thelight generating chamber (vessel) was directly incident on thefluorescent screen. FIG. 6A is a photograph of the fluorescent screenwhen the beam of EUV photons was passed through the original protectiontube before reaching the screen. As can be seen in FIG. 6A, there is awhite area in the middle of the screen. This white area is a “hot spot”that indicates grazing incidence reflections off the inner surface ofthe original protection tube contribute to the higher intensity in themiddle of the screen. FIG. 6B is a photograph of the fluorescent screenwhen the beam of EUV photons was passed through the roughened protectiontube before reaching the screen. As can be seen in FIG. 6B, there is no“hot spot” in the middle of the screen. Thus, the roughened protectiontube has essentially eliminated the contribution of grazing incidenceeffects and the intensity of light is substantially uniform across thescreen (as indicated by the uniform appearance of the screen that can beseen in FIG. 6B).

FIG. 7 is a flowchart diagram illustrating the method operations thatcan be used to roughen the inner surface of a protection tube, inaccordance with one embodiment of the invention. In operation 200, theinner surface of the protection tube is roughened using a firstroughening technique. The inner surface 142 x of protection tube 142(see FIGS. 3, 4A, and 4B) can be roughened using any suitable rougheningtechnique. For example, the inner surface can be roughened using a mediablasting technique, e.g., bead blasting, sandblasting, powder blasting,etc. The inner surface also can be roughened using a chemical technique,e.g., chemical etching, or a metal cutting technique, e.g., threading,milling, etc. Still further, the inner surface can be roughened using anabrasive technique, e.g., sanding, grinding, honing, electro-polishing,etc. As grazing incidence reflections can occur over the entirety of theinner surface of the protection tube, the roughening technique should beimplemented in a manner that ensures that the entirety of the innersurface will be roughened. In a case where the protection tube issituated so that grazing incidence reflections are not expected to occurover the entirety of the inner surface of the tube, it might besufficient to roughen only the portion (or portions) of the tube wheregrazing incidence reflections are expected to occur.

In operation 202, the inner surface of the protection tube is roughenedusing a second roughening technique. The second roughening technique canbe either the same as or different from the first roughening techniqueused in operation 200. As such, any of the roughening techniquesreferred to above in connection with operation 200 also can be used inoperation 202. Further, the use of the second roughening technique isoptional and can be omitted if the first roughening technique used inoperation 200 yields a roughened inner surface that is sufficient tominimize or essentially eliminate the effects of grazing incidencereflections.

In one example, the inner surface of the protection tube was roughenedby bead blasting in a first roughening process, and then was furtherroughened by threading the bead-blasted inner surface of the protectiontube in a second roughening process. The use of bead blasting as thefirst roughening technique ensures that the entire inner surface hasbeen roughened to some degree. This would not be the case if the innersurface of the protection tube was roughened only by threading becauseportions of the threaded inner surface, e.g., the tops of the individualthreads and the valley regions between the individual threads, mightstill be relatively smooth. As such, grazing incidences of light mightreflect off these relatively smooth surfaces. That said, depending onthe configuration of the threads and the size of the protection tube,roughening the inner surface of the protection tube using only threadingmight well be sufficient to minimize or essentially eliminate theeffects of grazing incidence reflections.

The degree to which the inner surface of the protection tube isroughened should to sufficient to cause grazing incidences of light tobe eliminated rather than to be reflected off the inner surface. Asdiscussed above, grazing incidence reflections occurred when the innersurface of the protection tube had a surface roughness of about 200nanometers but did not occur significantly when the inner surface had asurface roughness on the order of 300 microns. The surface roughness of300 microns is over 1,000 times larger than the surface roughness of 200nanometers; however, it is believed that such a large increase in thesurface roughness is not required to eliminate grazing incidences of EUVlight off the inner surface. Instead, it is believed that a surfaceroughness that is about 100 times larger than the original surfaceroughness of 200 nanometers should be sufficient to cause grazingincidences of light to be eliminated. Thus, in one example, the innersurface of the protection tube is roughened so as to have a surfaceroughness that is in the range from about 20 microns to about 1 mm(millimeter). The upper limit for the surface roughness is limited onlyby the degree of roughness that can be practically implemented in aprotection tube having the thickness on the order of those used inconnection with light generating chambers.

In another example, the inner surface of the protection tube isroughened so as to have a surface roughness in the range from about 50microns to about 0.5 millimeter. In yet another example, the innersurface of the protection tube is roughened so as to have a surfaceroughness in the range from about 100 microns to about 0.5 millimeter.In a further example, the inner surface of the protection tube isroughened so as to have a surface roughness in the range from about 200microns to about 0.1 millimeter.

FIG. 8 is a flowchart diagram illustrating the method operations thatcan be used to generate light, in accordance with one embodiment of theinvention. In operation 300, light is generated in a light generatingchamber. The light can be generated in any suitable light generatingchamber, e.g., vessel 102 described above with reference to FIG. 1. Thelight generated in the light generating chamber shown in FIG. 1 is EUVlight; however, it will be appreciated by those skilled in the art thatthe method is applicable to any wavelength of light. In operation 302, aportion of the light is caused to pass through a tube having a roughenedinner surface. As described above, a portion of the EUV photonsgenerated in vessel 102 shown in FIG. 1 is detected by the EUV PDmodules disposed external to the vessel. The generated EUV photons canbe caused to pass through a tube, e.g., protection tube 142 shown inFIGS. 2 and 3, by disposing the tube between the irradiation region 112(see FIG. 1) and an EUV PD module, e.g., EUV PD module PD-X shown inFIGS. 2 and 3. The inner surface of the tube can be roughened using anysuitable roughening technique, e.g., the method operations describedwith reference to FIG. 7. The inner surface of the tube should beroughened to a degree that is sufficient to cause grazing incidences oflight to be eliminated rather than to be reflected off the inner surfaceof the tube, as previously described.

In operation 304, the portion of the generated light that has passedthrough the tube is detected using a photodetector. In the examplesshown in FIGS. 1-3, the portion of the generated light (EUV photons)that has passed through the protection tube is detected by EUV PDmodules, each of which includes a sensor 152, which can be aphotodetector. The photodetector can be implemented using anyconfiguration suitable for detecting the generated light that has passedthrough the tube. As such, it will be appreciated by those skilled inthe art that the method of generating light can be implemented withoutusing the particular EUV PD modules described herein. In operation 306,a signal is output from the photodetector to a controller or otherdevice, e.g., a consuming system, a computer, memory, storage, and/or amonitoring computer. The signal corresponds to the portion of thegenerated light detected by the photodetector. The controller canquantify the signal using appropriate software and/or hardware todetermine the amount of the detected portion of the generated light.Based on this determination, the controller can cause any necessaryadjustments to be made to the parameters associated with generatinglight in the light generating chamber. By way of example, the controllercan adjust one or more of the following parameters: the timing of thesource laser; the power/amplification of the source laser; the optics ofthe source laser, e.g., the focusing and the steering of the laser; thesteering of the target material; and the timing of the target material.Once the signal has been output from the photodetector to thecontroller, the method is finished.

FIG. 9 is a flowchart diagram illustrating the method operations thatcan be used to measure light energy, in accordance with one embodimentof the invention. In operation 400, light is generated in a lightgenerating chamber. The light can be generated in any suitable lightgenerating chamber, e.g., vessel 102 described above with reference toFIG. 1. The light generated in the light generating chamber shown inFIG. 1 is EUV light; however, it will be appreciated by those skilled inthe art that the method is applicable to any wavelength of light. Inoperation 402, a portion of the light is caused to pass through a tubehaving a roughened inner surface. As described above, a portion of theEUV photons generated in vessel 102 shown in FIG. 1 is collected in theEUV PD modules disposed external to the vessel. The generated EUVphotons can be caused to pass through a tube, e.g., protection tube 142shown in FIGS. 2 and 3, by disposing the tube between the irradiationregion 112 (see FIG. 1) and an EUV PD module, e.g., EUV PD module PD-Xshown in FIGS. 2 and 3. The inner surface of the tube can be roughenedusing any suitable roughening technique, e.g., the method operationsdescribed with reference to FIG. 7. The inner surface of the tube shouldbe roughened to a degree that is sufficient to cause grazing incidencesof light to be eliminated rather than to be reflected off the innersurface of the tube, as previously described.

In operation 404, the portion of the generated light that has passedthrough the tube is detected using a photodetector. The photodetectorgenerates a signal corresponding to the portion of the generated lightdetected by the photodetector. The signal can be output directly to anindicator or other suitable display device and/or output to a processingdevice, e.g., a controller. In the examples shown in FIGS. 1-3, theportion of the generated light (EUV photons) that has passed through theprotection tube is detected by EUV PD modules, each of which includes asensor 152, which can be a photodetector. The photodetector can beimplemented using any configuration suitable for detecting the generatedlight that has passed through the tube. As such, it will be appreciatedby those skilled in the art that the method of measuring light energycan be implemented without using the particular EUV PD modules describedherein. Once the portion of the generated light that has passed throughthe tube has been detected, the method is finished.

Accordingly, the disclosure of the example embodiments is intended to beillustrative, but not limiting, of the scope of the inventions, whichare set forth in the following claims and their equivalents. Althoughexample embodiments of the inventions have been described in some detailfor purposes of clarity of understanding, it will be apparent thatcertain changes and modifications can be practiced within the scope ofthe following claims. In the following claims, elements and/or steps donot imply any particular order of operation, unless explicitly stated inthe claims or implicitly required by the disclosure.

What is claimed is:
 1. A method, comprising: generating light in a lightgenerating chamber; causing a portion of the generated light to passthrough a tube having a roughened inner surface; detecting the portionof the generated light that has passed through the tube using aphotodetector; and outputting a signal from the photodetector to acontroller, the signal corresponding to the detected portion of thegenerated light.
 2. The method of claim 1, wherein the generating oflight in the light generating chamber generates extreme ultraviolet(EUV) light.
 3. The method of claim 1, wherein the roughened innersurface of the tube has a surface roughness sufficient to cause grazingincidences of light to be eliminated rather than to be reflected off theroughened inner surface.
 4. The method of claim 1, wherein the innersurface of the tube is roughened by bead blasting the inner surface ofthe tube, and threading the bead-blasted inner surface of the tube. 5.The method of claim 1, wherein the roughened inner surface of the tubehas a surface roughness in a range from about 20 microns to about 1millimeter.
 6. The method of claim 1, wherein the roughened innersurface of the tube has a surface roughness in a range from about 100microns to about 0.5 millimeter.
 7. The method of claim 1, wherein theroughened inner surface of the tube has a surface roughness in a rangefrom about 200 microns to about 0.1 millimeter.
 8. A method, comprising:generating light in a light generating chamber; causing a portion of thegenerated light to pass through a tube having a roughened inner surface;and detecting the portion of the generated light that has passed throughthe tube using a photodetector.
 9. The method of claim 8, wherein thegenerating of light in the light generating chamber generates extremeultraviolet (EUV) light.
 10. The method of claim 8, wherein theroughened inner surface of the tube has a surface roughness sufficientto cause grazing incidences of light to be eliminated rather than to bereflected off the roughened inner surface.
 11. The method of claim 8,wherein the inner surface of the tube is roughened by bead blasting theinner surface of the tube, and threading the bead-blasted inner surfaceof the tube.
 12. The method of claim 8, wherein the roughened innersurface of the tube has a surface roughness in a range from about 20microns to about 1 millimeter.
 13. The method of claim 8, wherein theroughened inner surface of the tube has a surface roughness in a rangefrom about 100 microns to about 0.5 millimeter.
 14. The method of claim8, wherein the roughened inner surface of the tube has a surfaceroughness in a range from about 200 microns to about 0.1 millimeter.